Coin operated telephone stations or payphones are interfaced with a central office by a three wire loop circuit, consisting of tip, ring and ground lines. In one type of commonly practiced fraud, known as "pin fraud", the wires in the handset of a payphone are manipulated so that the payphone's coin escrow bucket cannot be activated to collect or refund escrowed coins. In addition, this fraud causes the establishment of the initial rate needed for a local call. As a result, a vandal may make unlimited local calls without depositing any coins.
Furthermore, if customers deposit coins in a "pinned" telephone then these coins will collect in the escrow bucket. After the passage of time, the vandal returns to the pay telephone and reverses the fraud. Since the default mode of the phone is to refund coins sensed in the escrow bucket when no payment is due, the escrow bucket then releases the accumulated coins to the refund chute. The vandal takes the coins, thus defrauding the owner of the pay telephone. Thus, this fraud causes both a loss of revenues to the phone's owner, as well as customer dissatisfaction and ill-will when the customer does not receive a refund that he is entitled to such as when a call to the operator is made or the customer hangs up before the call is connected.
The method used to perpetrate this type of telephone fraud frequently causes damage to the pay telephone itself. The connecting cable and handset are often irreparably damaged by the vandal. This property damage is expensive to fix, both in terms of the costs of parts and labor. Many thousands of payphones, mostly in major cities, are damaged each year by vandals practicing this fraud. It is hoped that removing the incentive to practice this method of defrauding pay telephones will deter vandals from attempting it and thus keep them from damaging the telephones.
In the past, a fuse has been added to the telephone control circuit so that when a pin fraud is attempted the fuse blows. See U.S. patent application Ser. No. 07/199,129 filed May 26, 1989 and assigned to the assignee of the present invention. Firstly, the fuse thwarts the fraud by eliminating the possibility of unlimited free calls. Secondly, if a customer picks up the handset to make a call at a time when the fuse has been blown, he finds that the phone is not operational as no dial tone is heard. In this case, the customer will hopefully follow the operational instructions and not use the phone. Thirdly, and more importantly, if the customer inadvertently deposits coins with the fuse blown, they will be refunded to him, and will not be refunded to the vandal. Consequently, the incentive for performing the fraud is removed. However, the pay telephone is put out of service by the blown fuse and requires a service call to replace the fuse before the phone can be put back in service.
In an alternative prior art approach described in U.S. Pat. No. 4,845,742, one isolation transformer is used to isolate a receiver speaker element and a second isolation transformer is used to isolate a dynamic microphone. The use of an isolation transformer to isolate a receiver speaker was routinely used long prior to U.S. Pat. No. 4,845,742. See, for example, GTE, Coin Telephone Handbook, Section 476-201100 at p. 25/26, August 1978. Similarly, the use of an isolation transformer to isolate an audio input was also known prior to U.S. Pat. No. 4,845,742. See, for example, the isolation of the auxiliary audio input of FIG. 21 of U.S. application Ser. No. 199,129.
This well known isolation technique of using an isolation transformer cannot be used with a carbon microphone because such a microphone requires a DC bias to operate. Further, the use of an isolation transformer as shown in U.S. Pat. No. 4,845,742 requires that the microphone be isolated all the time. For testing the presence or absence of the handset or adjusting the volume of the speaker in the earpiece as taught in U.S. application Ser. No. 199,129, it is desirable to connect and disconnect the microphone at the appropriate times rather than to permanently isolate it.